Cartridge-based water filter

ABSTRACT

A fluid-filtering apparatus includes an upper cartridge and a lower cartridge. The upper cartridge includes a fluid-filtering first tube that surrounds and extends along an axis and that defines a first-tube cavity. An axially-extending fluid-filtering second tube is located within the first-tube cavity and defines a second-tube cavity. The lower cartridge includes an axially-extending third tube that defines a third cavity. An axially-extending fluid-filtering fourth tube, of the lower cartridge, is located within the third cavity and defines a fourth-tube cavity. The upper and lower cartridges are coupled together and contained within a casing.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. patent application Ser. No. 15/004,026,filed on Jan. 22, 2016, the entirety of which is incorporated herein byreference.

TECHNICAL FIELD

This relates to cartridge-based water filters.

BACKGROUND

A water filter is used to filter water. The filter may include a seriesof filter cartridges through which the water flows. Each cartridge inthe series filters out contaminants from the water flowing through it.

SUMMARY

In one aspect, a fluid-filtering apparatus includes an upper cartridgeincluding a first tube of a first fluid-filtering material, thatsurrounds and extends along an axis and that defines a first-tube cavityextending from a closed top to a first-tube bottom opening; anaxially-extending second tube of a second fluid-filtering material, thatis located within the first-tube cavity and that defines a second-tubecavity extending from a closed top to a second-tube bottom opening; andan upper-cartridge bottom cap that extends across, and seals, thefirst-tube bottom opening; a lower cartridge including anaxially-extending third tube, that defines a third-tube cavity with athird-tube top opening that communicates with the second-tube bottomopening; an axially-extending fourth tube comprising a fourthfluid-filtering material, that is located within the third-tube cavityand that defines a fourth-tube cavity that extends from a closed top endto a fourth-tube bottom opening; and a lower-cartridge bottom cap thatextends across, and seals, the second-tube bottom opening; and a casingincluding an inlet channel configured to channel fluid into thefluid-filtering apparatus, an outlet channel configured for fluidcommunication with the fourth-tube bottom opening.

Fluid flowing through the fluid-filtering apparatus may be configured toflow in an order of (i) into the casing through the inlet channel, (ii)through and into the first tube, (iii) through and into the second tube,(iv) downward through the second-tube bottom opening and into thethird-tube top opening, (v) through and into the fourth tube, (vi)through the fourth-tube bottom opening, and (vii) out through thecasing's outlet channel.

In another aspect, a method includes filtering water using thefluid-filtering apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an example filtering apparatus, shown inan assembled configuration.

FIG. 2 is an exploded (unassembled) view of the apparatus.

FIG. 3 is an expanded view of certain sections of apparatus.

FIGS. 4A-4C are different views, taken from different viewpoints, of afirst tube of the apparatus.

FIGS. 5A-5B are different views, taken from different viewpoints, of asecond tube of the apparatus.

FIGS. 6A-6D are different views, taken from different viewpoints, of atop cap located on the second tube.

FIGS. 7A-7D are different views, taken from different viewpoints, of abottom cap located under the first and second tubes.

FIGS. 8A-8C are different views, taken from different viewpoints, of athird tube of the apparatus.

FIGS. 9A-9D are different views, taken from different viewpoints, of atop cap located on the third tube.

FIGS. 10A-10B are different views, taken from different viewpoints, of afourth tube of the apparatus.

FIGS. 11A-11C are different views, taken from different viewpoints, of atop cap located on the fourth tube.

FIGS. 12A-12D are different views, taken from different viewpoints, of abase of a casing of the apparatus.

FIGS. 13A-13C are different views, taken from different viewpoints, of atube of the casing.

FIGS. 14A-14D are different views, taken from different viewpoints, of apositioning disk of the apparatus.

FIG. 15A is a sectional view, showing where glue is applied to the firstand second tubes.

FIG. 15B is a sectional view, showing where glue is applied in the thirdand fourth tubes.

FIG. 16 is a sectional view showing a fluid flow path in the apparatus.

DETAILED DESCRIPTION

FIG. 1 is a sectional view of an example fluid filtering apparatus 1,shown in an assembled configuration. FIG. 3 is an exploded (unassembled)view of the apparatus 1. FIG. 2 is an expanded view of certain sectionsof apparatus 1.

The apparatus 1 includes an upper cartridge 2, and a lower cartridge 3.The cartridges 2, 3 are screwed (coupled) together and housed in acasing 4. In operation, fluid flows through an inlet 5 of the casing 4,through filtering components of the upper cartridge 2 and the lowercartridge 3, and out an outlet 6 of the casing. The fluid being filteredby the apparatus may be a liquid, for example water.

The example apparatus 1 is described below with reference to an axis A.With reference to the apparatus' orientation in FIGS. 1-3, axis A isvertical. Each component of the example apparatus 1 is centered on theaxis A. In the description below: “Axially” means along axis A.“Laterally” means perpendicular to axis A. “Upper” and “lower” are withreference to the apparatus' orientation in FIG. 1. Accordingly, if theapparatus 1 would be oriented upside down, the “upper” cartridge 2 wouldbe below the “lower” cartridge 3. And if the apparatus 1 would beoriented sideways (on its side), the “upper” and “lower” cartridges 2, 3would be on the same level.

FIGS. 3 and 4A-4C show an axially-extending permeable rigid first tube10 of upper cartridge 2. The first tube 10 is formed of a firstfluid-filtering material. The first fluid-filtering material may bepermeable porous ceramic. The first tube 10 includes a tubular section14 and an adjoining crown 10T, both comprised of the firstfluid-filtering material and formed as a one-piece unit. The first tube10 defines a first-tube cavity 11 that extends downward from a closedtop (crown 10T) to a first-tube bottom opening 11B. The crown 10Toverlies and extends across the first-tube cavity 11, and the tubularsection 14 surrounds and bounds the cavity 11.

FIGS. 3 and 5A-5B show an axially-extending permeable rigid second tube20 of the upper cartridge 2. The second tube 20 is formed of a secondfluid-filtering material. This second fluid-filtering material may bepermeable porous activated carbon. The second tube 20 is located withinthe first-tube cavity 11. The second tube 20 defines a second-tubecavity 21, extending from a second-tube bottom opening 21B to asecond-tube top opening 21T.

FIGS. 3 and 6A-6D show an impermeable rigid second-tube top cap 23 thatextends across, and seals off, the second-tube top opening 21T. Acircumferential (circumferentially-extending) wall of the second tube 20is captured between a radially outer wall 23 a and a radially inner wall23 b of the second-tube top cap 23.

A circumferential upper intermediate space 24 is located within thefirst-tube cavity 11 and circumferentially surrounds the second tube 20.The upper intermediate space 24 extends axially upward from an upperintermediate bottom opening 24B.

FIGS. 3 and 7A-7D show an impermeable upper-cartridge bottom cap 27 thatextends across, and seals, the annular bottom opening 24B of the upperintermediate space 24. The upper-cartridge bottom cap 27 includes athreaded upper-cartridge outlet port 28 through which fluid may exit thesecond-tube cavity 21.

FIGS. 3 and 8A-8C show an axially-extending impermeable third tube 30 ofthe lower cartridge 3. The third tube 30 may be of rigid plastic (e.g.,PVC, i.e., polyvinyl chloride). The third tube defines a third-tubecavity 31 with a third-tube top opening 31T and a third-tube bottomopening 31B. The third-tube top opening 31T communicates withsecond-tube bottom opening 21B, in that fluid exiting the second-tubebottom opening 21B will flow through the third-tube top opening 31T.

FIGS. 3 and 9A-9D show an axially-extending impermeable rigid third-tubetop cap 33 that extends across the third tube top opening 31T. Thethird-tube top cap 33 has an internally-threaded lower-cartridge inletport 34 that is configured for threaded engagement with the threadedupper-cartridge outlet port 28. This enables fluid to flow from thesecond-tube cavity 21, through the upper-cartridge outlet port 28 andthe lower-cartridge inlet port 34, into the third-tube cavity 31. Acircumferentially-extending wall of the third tube 30 is capturedbetween a radially-outer wall 33 a and a radially-inner wall 33 b of thethird-tube top cap 33.

FIGS. 3 and 10A-10B show an axially-extending permeable rigid fourthtube 40 of the lower cartridge 3. The fourth tube 40 is formed of afourth liquid-filtering material. The fourth liquid-filtering materialmight be of permeable porous activated carbon. The fourth tube 40 islocated within the third-tube cavity 31. The fourth tube 40 defines afourth-tube cavity 41 that extends axially from a fourth-tube topopening 41T to a fourth-tube bottom opening 41B.

FIGS. 3 and 11A-10C show a fourth-tube top cap 44 that extends across,and seals, the fourth-tube top opening 41T. The fourth tube 40 iscaptured by a circumferentially-extending peripheral wall 44 a of thefourth-tube top cap 44.

As shown in FIG. 3, a circumferential lower intermediate space 45 islocated within the third-tube cavity 31 and circumferentially surroundsthe fourth tube 40.

An impermeable lower-cartridge bottom cap 47 in this example isidentical to the upper-cartridge bottom cap 27 shown in FIGS. 7A-7D. Asshown in FIG. 3, the lower-cartridge bottom cap 47 extends across, andseals, an annular bottom opening of the lower intermediate space 46. Thelower-cartridge bottom cap 47 includes an externally-threadedlower-cartridge outlet port 48 through which fluid may exit the fourthtube 40. All caps 23, 27, 33, 44, 47 in this example are of plastic(e.g, polyethylene).

The cartridges 2, 3 are contained in a casing cavity 51 that is enclosedby the casing 4 (housing, container). The casing 4 is formed of a rigidmaterial (e.g., stain steel, plastic). A casing intermediate space 52 islocated within the casing cavity 51 and circumferentially surrounds thecartridges 2, 3. The inlet channel 5 in the casing 4 channels fluid intothe casing cavity 51 (container cavity). The outlet channel 6 in thecasing 4 channels fluid out of the casing cavity 51.

FIGS. 3 and 12A-12D show a base 60 of the casing 4. The base 60 has aclosed bottom and a top opening 62. A base recess 63 (cavity) extendsdownward from the base's top opening 62. A cylindrical sidewall 64surrounds and bounds the casing recess 63. The base's sidewall 64 has aninternal screw thread 65.

FIGS. 3 and 13A-13C show a casing tube 70 of the casing 4. The casingtube 70 has a closed top 71 (crown) and a bottom opening 72. The bottomopening 72 is surrounded by an annular bottom edge 73. An external screwthread 75 extends around an external surface of the casing tube 70. Thecasing tube's screw thread 75 is screwed, by the end user, into thebase's screw thread 65. The casing's base 60 and tube 70 are thusscrewed together, and together encase the casing cavity 51.

FIGS. 3 and 14A-14D show a positioning disk 80. The positioning disk 80in this example is of rigid plastic (e.g., PVC, polyethylene). It helpskeep the cartridges 2, 3 centered in the casing cavity 51. Thepositioning disk 80 has an annular periphery 81 that is pressed down bythe casing tube's bottom edge 73. The lower cartridge'sexternally-threaded outlet port 48 is screwed into an axially-extendinginternally-threaded bore 82 of the positioning disk 80. The positioningdisk 80 has axially-extending channels 88 (through-holes) to enablefluid below the disk 80 to flow upward through the disk 80. This examplehas four channels 88 spaced circumferentially 90 degrees apart from eachother. Another example might have two channels spaced circumferentially180 degrees apart from each other. Flow of the fluid (water) through theapparatus 1 might be a positive function of the number of channels 88and the diameters of the channels 88. Accordingly, the number anddiameters of the channels 88 might be used (selected by a designer ofthe apparatus) to help regulate the flow through the apparatus 1.

As shown in FIGS. 3 and 12D, a flat bottom surface 83 of the casing base60 is interrupted by a cylindrical depression 84 into which the lowercartridge's outlet port 48 extends. An O-ring 124 is compressed betweenan annular distal end of the lower cartridge's outlet port 48 and abottom surface 86 of the cylindrical depression 84, and is preventedfrom moving laterally by the depression's side surface. The casing base60 applies, through intermediacy of the O-ring 124 and the lowercartridge's outlet port 48, upward pressure on positioning disk 80.

The positioning disk 80 is axially fixed relative to the casing 4 byabutment from above from the casing tube's bottom edge 73 and byabutment from below from the base 60. The positioning disk 80 islaterally fixed relative to the casing 4 by abutment of disk's annularperiphery 81 with the base's sidewall 64.

As shown in FIGS. 3 and 15B, the positioning disk's axial position keepsthe positioning disk 80 raised above the bottom surface 83 of the baserecess 63, leaving a flow space 87 between the positioning disk 80 andthe casing's bottom surface 83. Fluid in the flow space 87 can flowupward through the channels 88 in the positioning disk 80.

Referring to FIGS. 7D and 15A-15B, the bottom caps 27, 47 of the upperand lower cartridges 2, 3 are identical. Each bottom cap 27, 47 includesa laterally-extending top surface 90 that is interrupted by five annularbosses 91-95. The annular bosses 91-95 project upward from the topsurface 90 and extend circumferentially about the axis A and arecentered on the axis A. The first boss 91 is an outer wall located alongthe periphery of the bottom cap 27. Each of the second, third, fourthand fifth bosses 92, 93, 94, 95 is of successively smaller radius. Thefifth boss 95 is an upwardly-projecting extension of the upper-cartridgeoutlet port 28, and surrounds and bounds an outlet channel of the lowercartridge 3. The first and fifth bosses 91, 95 are approximately thesame height (about 0.4 inch tall in this example), and are taller thanthe second, third and fourth bosses 92, 93, 94 (which are about 0.15inch tall in this example).

As shown in FIG. 15A, regarding the upper cartridge's bottom cap 27: Thefirst tube 10 is received closely by the first boss 91 and rests on topof the second boss 92. The second tube 20 rests on the fourth boss 94and closely receives (engages) the fifth boss 95.

As shown in FIG. 15B, regarding the lower cartridge's bottom cap 47: Acircumferential wall of the third tube 30 is captured between (closelyreceived by) the first and second bosses 91, 92, The fourth tube 40rests on the fourth boss 94 and closely receives (engages) the fifthboss 95.

As shown in FIG. 16, a flexible top spacer 100 (centering piece) isadheringly connected to a top end of the second tube 20. In thisexample, the top spacer 100 is a piece of sponge. The adheringconnection is indirect, in that the spacer 100 is adhered to a topsurface of the second-tube top cap 23 which is itself adhered to thesecond tube 20. The spacer 100 non-adheringly abuts an inner surface ofthe first tube's crown 10T. In this example, the sponge 100 ishemispherical, and the non-adhering abutment extends substantially alongthe entire hemispherical outer surface of the spacer 100 and the entirehemispherical inner surface of the crown 15. Alternatively, the spacer100 might be disk-shaped with an annular peripheral surface (rim) thatnon-adheringly abuts an inner surface of the first tube 10. When theapparatus 1 is assembled, the non-adhering abutment reduces lateralmovement of the distal end of the second tube 20 relative to the firsttube 10 when the upper cartridge 2 is shaken. During assembly, thenon-adhering abutment additional reduces the probability of the distalend of the second tube 20 scraping the inner surface of the first tube10 when being inserted into the first tube 10.

Certain parts are permanently adhered together—in this example with glue(e.g., hot-melt glue). In FIGS. 15A-15B and 16, examples of pools ofglue (glue of significant thickness) are portrayed with black shading.Examples of glue with negligible thickness, located between adjacentwalls that abut or almost abut each other are portrayed with thickdashed lines.

As shown in FIG. 15A, glue adheres both the first tube 10 and the secondtube 20 to the lower cartridge's bottom cap 27. Glue extends from abottom surface 200 of the first tube 10 to both (i) a radially-outerside surface of the second boss 92 and (ii) a radially-inner sidesurface of the second boss 92. Glue also extends from a bottom surface210 of the second tube 20 to both (i) a radially-outer side surface ofthe fourth boss 94 and (ii) a radially-inner side surface of the fourthboss 94. This configuration, of glue adhering to both inner and outerside surfaces of bosses 92, 94, may provide adhesion that is strongerand more reliable than if the caps' top surfaces were flat and lackedthe bosses.

As shown in FIG. 15B, glue adheres both the third tube 30 and the fourthtube 40 to the lower cartridge's bottom cap 47. Glue extends from abottom surface 220 of the fourth tube to (i) a radially-outer sidesurface of the fourth boss 94 and (ii) a radially-inner side surface 222of the fourth boss 94.

As shown in FIG. 3, glue adheres the flexible spacer 100 to thesecond-tube top cap 23, and adheres the second-tube top cap 23 to thesecond tube 20, and adheres the third-tube top cap 33 to the third tube30, and adheres the fourth-tube top cap 44 to the fourth tube 40.

As shown in FIG. 3, O-rings provide a seal between components. A firstO-ring 121 provides a seal between the upper cartridge 2 and the lowercartridge 3. A second O-ring 122 provides a seal between the lowercartridge 3 and the positioning disk 80. A third O-ring 123 provides aseal between the positioning disk 80 and the casing base 60. A fourthO-ring 124 provides a seal between the lower cartridge's outlet port 48and the casing base 60. A fifth O-ring 125 provides a seal between thecasing base 60 and the casing tube 70.

The apparatus 1 may be sold to an end user in four user-separableparts—the upper cartridge 2, the lower cartridge 3, the casing base 60and the casing tube 70. The apparatus 1 may be sold and provided by avendor to the user in the assembled configuration (shown in FIG. 1).Alternatively, the apparatus 1 may be provided to the user in anunassembled configuration of the four user-separable parts. To assemblethe apparatus 1, the user can screw the upper cartridge 2 into the lowercartridge 3, then screw the lower cartridge 3 into the positioning desk80, then position the cartridges 2, 3 over the casing base 60 such thatthe lower cartridge's outlet port 3 is inserted into the casing base'srecess 84, then move the casing tube 70 down over the cartridges 2,3,and screw the casing tube 70 into the casing base 60.

FIG. 16 shows a fluid flow path through the apparatus 1. All fluidentering the casing cavity 51 through the casing inlet channel 5 may beconstrained by the apparatus 1 to (i) flow into the flow space 87between the positioning disk 80 and the casing's bottom surface 83, (ii)upward through the holes 88 in the disk, (iii) upward through the casingintermediate space 52, (iv) radially-inward through the first tube 10and through the second tube 20 into the second-tube cavity 21, (v)downward through the upper-cartridge outlet port 28 and thelower-cartridge inlet port 34, (vi) into the third-tube cavity 31 andthe lower intermediate space 45, (vii) radially inward through thefourth tube 40 into the fourth-tube cavity 41, (viii) downward throughthe lower-cartridge outlet port 48 and (iv) through the casing outletchannel 6 to exit the casing 4.

A manufacturer may manufacture a variety of upper cartridges 2 that areinterchangeable with each other and differ from each other in terms oflength and filtering materials used for the first and second filteringtubes 10, 20. The manufacturer may also manufacture a variety of lowercartridges 3 that are interchangeable with each other in terms of lengthand filtering materials used for the fourth filtering tube 40. Some ofthe available upper cartridges may be shorter than some of the availablelower cartridges, and longer than others. The choice of which uppercartridge and which lower cartridge to use may depend on which cartridgelength (tube length) and which filtering materials are best suited forfiltering out the particular contaminants and contaminant concentrationsfound in the end user's water. Although the above example uses just onelower cartridge 3, different lower cartridges may be screwed togetherend-to-end in series, between the upper cartridge 2 and the positioningdisk 80, as long as the overall length of the cartridges is short enoughfor the cartridges to fit in the casing 4.

Although the fluid being filtered in this example is water, the fluidmay alternatively be a gas or a liquid other than water.

The components and procedures described above provide examples ofelements recited in the claims. They provide examples of how a person ofordinary skill in the art can make and use the claimed invention. Theyare described here to provide enablement without imposing limitationsthat are not recited in the claims. In some instances in the abovedescription, a term is followed by an alternative term, or asubstantially equivalent term, enclosed in parentheses.

The invention claimed is:
 1. A fluid-filtering apparatus, comprising: anupper cartridge including: a first tube of a first fluid-filteringmaterial, that surrounds and extends along an axis and that defines afirst-tube cavity extending from a closed top to a first-tube bottomopening; an axially extending second tube of a second fluid-filteringmaterial, that is located within the first-tube cavity and that definesa second-tube cavity extending from a closed top to a second-tube bottomopening; and an upper-cartridge bottom cap that extends across, andseals, the first-tube bottom opening; a lower cartridge including: anaxially-extending third tube, that defines a third-tube cavity with athird-tube top opening that communicates with the second-tube bottomopening; an axially-extending fourth tube comprising a fourthfluid-filtering material, that is located within the third-tube cavityand that defines a fourth-tube cavity that extends from a closed top endto a fourth-tube bottom opening; and a lower-cartridge bottom cap thatextends across, and seals, the second-tube bottom opening; and a casingincluding: an inlet channel configured to channel fluid into thefluid-filtering apparatus; and an outlet channel configured for fluidcommunication with the fourth-tube bottom opening, wherein fluid flowingthrough the fluid-filtering apparatus is configured to flow in an orderof (i) into the casing through the inlet channel, (ii) through and intothe first tube, (iii) through and into the second tube, (iv) downwardthrough the second-tube bottom opening and into the third-tube topopening, (v) through and into the fourth tube, (vi) through thefourth-tube bottom opening, and (vii) out through the casing's outletchannel.
 2. The apparatus of claim 1, wherein the upper-cartridge bottomcap includes a threaded upper-cartridge outlet port through which fluidmay exit the second-tube cavity.
 3. The apparatus of claim 2, whereinthe lower cartridge includes a third-tube top cap that (i) extendsacross the third-tube top opening and that (ii) has a threadedlower-cartridge inlet port that is configured for threaded engagementwith the threaded upper-cartridge outlet port.
 4. The apparatus of claim1, wherein the second tube includes an impermeable top cap that overliesand extends across the second-tube cavity.
 5. The apparatus of claim 1,further comprising a flexible top spacer, adheringly connected to a topend of the second tube, and configured to non-adheringly abut an innersurface of the first tube.
 6. The apparatus of claim 5, wherein theadhering connection is through the top spacer being glued to a cap thatis glued to a top of the second tube.
 7. The apparatus of claim 1,wherein the lower-cartridge bottom cap includes a threadedlower-cartridge outlet port that allows fluid to exit the fourth-tubecavity.
 8. The apparatus of claim 1, wherein the casing includes acasing base that has (i) a top opening, (ii) a casing cavity extendingdownward from the base's top opening, and (iii) an internal screw threadsurrounding the base's cavity; and a casing tube that has (i) a closedtop end, (ii) a bottom opening, (ii) an annular bottom edge surroundingthe casing tube's bottom opening, (iii) a casing cavity, extendingupward from the bottom opening, and (iv) an external screw thread thatis external to casing cavity and surrounds the casing cavity and isconfigured to be screwed into the base's screw thread, for the casingbase and the casing tube to be screwed together and together bound thecasing cavity.
 9. The apparatus of claim 8, wherein the apparatusfurther comprises a positioning disk having (i) a periphery configuredto be pressed down by the casing tube's bottom edge, and (ii) anaxially-extending threaded bore, and wherein the lower-cartridge bottomcap includes a threaded lower-cartridge outlet port configured to bescrewed into the positioning disk's threaded bore.
 10. The apparatus ofclaim 9, wherein the casing base is configured to apply upward pressureon the lower-cartridge outlet port to keep the positioning disk raisedabove a bottom surface of the base cavity.
 11. The apparatus of claim 9,wherein the positioning disk includes axially-extending channels thatenable fluid exiting the casing inlet to flow upward through thepositioning disk.
 12. The apparatus of claim 9, wherein the casingbase's bottom surface includes a cylindrical depression into which thelower-cartridge outlet port extends, and wherein a bottom surface of thedepression applies upward pressure on the lower-cartridge outlet port.13. The apparatus of claim 9, wherein the positioning disk is axiallyfixed relative to the casing (i) by abutment from above from the casingtube's bottom edge against the positioning disk and (ii) by abutmentfrom below from the base against the lower-cartridge outlet port. 14.The apparatus of claim 1, wherein the upper-cartridge bottom capincludes a laterally-extending top surface that is interrupted byannular first, second, third and fourth bosses that project upward fromthe top surface and extend circumferentially about axis, and wherein thefirst tube is received closely by the first boss and rests on the secondboss, and the second tube rests on the third boss and closely receivesthe fourth boss.
 15. The apparatus of claim 14, wherein: glue extendsfrom a bottom surface of the first tube to a radially-outer side surfaceof the second boss; glue extends from the bottom surface of the firsttube to the radially-inner side surface of the second boss; glue extendsfrom a bottom surface of the second tube to a radially-outer sidesurface of the third boss; and glue extends from the bottom surface ofthe second tube to a radially-inner side surface of the third boss. 16.The apparatus of claim 1, wherein the lower-cartridge bottom capincludes a laterally-extending top surface that is interrupted byannular first, second, third and fourth bosses that project upward fromthe top surface and extend circumferentially about axis, wherein thethird tube comprises a tubular wall that is captured closely between thefirst and second bosses, and the fourth tube rests on the third boss andclosely receives the fourth boss.
 17. The apparatus of claim 1, whereinthe first fluid-filtering material comprises ceramic, and both thesecond fluid-filtering material and the third fluid-filtering materialcomprise carbon.
 18. The apparatus of claim 1, wherein the first,second, third and fourth tubes have a cylindrical periphery centered onthe axis, and the bottom caps have a cylindrical periphery.
 19. Theapparatus of claim 1, wherein the lower cartridge includes a singlefourth tube within the third-tube cavity.
 20. A method, comprisingfiltering water using the apparatus of claim 1.